when docviewer has scrollbars, don't let keyboard scrolling events propagate
fix comments
handle scrolling for any element in viewer
optimize code that sets scrollingUp
fix scrolling with focus inside viewer, but outside scroller
Merge remote-tracking branch 'origin/master' into randy/issue-5884
provide customViewerRegistry to enable extension developers to add CustomViewers
don't use global keydown hook
fix tabbing in content
review feedback
 register ImageViewer more elegantly.
Merge pull request #6017 from adobe/randy/issue-5884

When InlineDocViewer has scrollbars, don't let keyboard scrolling events propagate
Merge pull request #6172 from adobe/couzteau/custom-viewer-registry

customViewer API: Registry to enable extension developers to add CustomViewers